1. Field of the Invention
The present invention generally relates to carpets, and more specifically to a carpet suited for preventing the transmission of vibration and sound into a vehicle cabin.
2. Description of the Prior Art
In order to shut out noise and bibration from the vehicle cabin, it is a common practice to insulate the various panels and components, such as the dashpanel, the roof, floor and the like. In addition, it is common to use rubber sheeting, felt carpet and the like, and to arrange the vibration and noise attenuating characteristics of each of the additional components and elements in a manner which, in combination with the above mentioned panels and components, produces a tuned system, the overall effect of which is reduce the unwanted vibration/noise to the degree where even minute vibrations are shut out.
JU-A-61-187757 discloses a carpet construction of the nature shown in FIG. 6. In this figure, a carpet 1 is laid on top of a metallic vehicle floor panel 2. The carpet, by way of example, comprises a relative rigid polyurethane layer 3 on which an upper decorative layer 5 is disposed.
The vibration and noise which is transmitted from the external surface 2a of the floor panel to the internal one 2b, is attenuated/shut out by the polyurethane layer 3 which in this instance exhibits a rigidity K of about 2.5.times.10.sup.-4 /mm.sup.3, and the decorative flexible layer 5 which exhibits a density of about 1-2 Kg/m.sup.3.
With this arrangement, the noise which is experienced by the passenger(s) seated in the front seat(s) of the vehicle is indicated by the ratio of the sound pressure as measured at the front seat(s) and the exciting noise pressure.
FIG. 9 shows the noise characteristics which are experienced in a given type of vehicle cabin wherein no carpet is laid on the floor panel, while FIG. 11 shows the characteristics which are obtained in the same environment but with the above mentioned prior art type of carpet laid.
As will be appreciated from FIG. 11 wherein C1 denotes the characteristics which are produced when the vehicle occupants feet do not rest on the surface of the carpet while C2 denote the characteristics which occur when a passenger or passengers feet rest on top of the carpet. As the characteristics shift quite notably, it is difficult to accurately achieve the desired combination of vibration/noise damping characteristics which are required to achieve a system which will achieve the maximum amount of noise/vibration attenuation.
More specifically, investigations have shown that if a mass M which is representative of the passenger's feet 7, is placed on the upper surface of the carpet 1 in manner to apply a force of about 280 Kg/m.sup.2, the characteristics with which vibration is transmitted between the surfaces 2a and 2b shifts, as shown in FIG. 8, from the broken line trace denoted by D2 to that denoted by the solid line trace D1 in a manner wherein the frequency at which the peak transmission ratio thus changes markedly.
In addition, it should be noted that, as shown in FIG. 9 (depicting the case wherein no carpet is laid), the frequency range denoted by A follows a sharp depression T and occurs in the 200-300 Hz range and that, in order to achieve the broadest reduction in the sound pressure in the front seat zone, the carpet is designed to damp the sound in frequency range A. In other words, arranged the carpet is to attenuate noise in the range which follows from one in which noise and vibration radiation is naturally low.
It should also be noted that the prior art carpet is designed so that, as shown in FIG. 10, the transmitivity thereof (viz., the transmitted sound pressure/ the exciting sound pressure) exhibits the characteristics denoted by trace B1 and wherein the peak value P1 is arranged to occur at the same frequency as the marked reduction T. The frequency at which this peak occurs can be derived from the following equation: ##EQU1##
However, when a passenger's feet rest on the carpet, as shown in FIG. 7, the carpet as a whole, or at least large areas thereof, become compressed and the transmitivity of the carpet changes from the characteristics denoted by B1 to those denoted by B2. As a result the frequency at which peak transmitivity value changes from P1 to P2 by about 50-100 Hz. In other words, if the passenger's feet are represented by the above mentioned mass M then the situation wherein: ##EQU2##
As a result, the tuning which was achieved before the passenger's feet came to rest on the carpet is disturbed in the manner depicted by trace C2 in FIG. 11 and gives to the generation of reverberation noise.